The present invention relates to vertical power semiconductor devices including active devices such as MOSFETs, IGBTs and bipolar transistors, and passive devices such as diodes. Specifically, the present invention relates to vertical power super-junction semiconductor devices which facilitate realizing a high breakdown voltage and a high current capacity.
The semiconductor devices may be classified into a lateral device, which arranges the main electrodes thereof on one major surface, and a vertical device, which distributes the main electrodes thereof on two major surfaces facing opposite to each other. In the vertical semiconductor device, a drift current flows in the thickness direction of the semiconductor chip (vertically) in the ON-state of the semiconductor device and depletion layers expand also in the thickness direction of the semiconductor chip (vertically) in the OFF-state of the semiconductor device.
FIG. 9 is a cross sectional view of a conventional planar-type n-channel vertical metal oxide semiconductor field effect transistor (MOSFET). Referring now to FIG. 9, the vertical MOSFET includes a drain electrode 18 on the back surface of a semiconductor chip; an n+-type drain layer 11 with low electrical resistance in electrical contact with drain electrode 18; a very resistive nxe2x88x92-type drain drift layer 12 on n+-type drain layer 11; p-type base regions 13 formed, as channel diffusion layers, selectively in the surface portion of nxe2x88x92-type drain drift layer 12; a heavily doped n+-type source region 14 formed selectively in the surface portion of p-type base region 13; a heavily doped p+-type contact region 19 formed selectively in the surface portion of p-type base region 13 for realizing ohmic contact; a polycrystalline silicon gate electrode layer 16 above the extended portion of p-type base region 13 extended between n+-type source region 14 and very resistive nxe2x88x92-type drain drift layer 12 with a gate insulation film 15 interposed therebetween; and a source electrode layer 17 in contact with n+-type source regions 14 and p+-type contact regions 19. In the vertical semiconductor device as described above, nxe2x88x92-type drain drift layer 12 works as a layer, through which a drift current flows vertically in the ON-state of the MOSFET. In the OFF-state of the MOSFET, nxe2x88x92-type drain drift layer 12 is depleted by the depletion layers expanding in the depth direction thereof (vertically) from the pn-junctions between nxe2x88x92-type drain drift layer 12 and p-type base regions 13, resulting in a high breakdown voltage.
Thinning very resistive nxe2x88x92-type drain drift layer 12, that is shortening the drift current path, facilitates substantially reducing the on-resistance (the resistance between the drain and the source), since the drift resistance in the ON-state of the semiconductor device is reduced. However, thinning very resistive nxe2x88x92-type drain drift layer 12 narrows the width between the drain and the base region, for which depletion layers expand from the pn-junctions between nxe2x88x92-type drain drift layer 12 and p-type base regions 13. Due to the narrow expansion width of the depletion layers, the depletion electric field strength soon reaches the maximum (critical) value for silicon. Therefore, breakdown is caused at a voltage lower than the designed breakdown voltage of the semiconductor device. A high breakdown voltage is obtained by thickening the nxe2x88x92-type drain drift layer 12. However, a thick nxe2x88x92-type drain drift layer 12 inevitably causes high on-resistance, which further causes on-loss increase. In other words, there exists a tradeoff relation between the on-resistance (current capacity) and the breakdown voltage. The tradeoff relation between the on-resistance (current capacity) and the breakdown voltage exists in other semiconductor devices, which include a drift layer, such as IGBTs, bipolar transistors and diodes. The tradeoff relation between the on-resistance (current capacity) and the breakdown voltage also exists in lateral semiconductor devices, in which the flow direction of the drift current in the ON-state and the expansion direction of the depletion layers in the OFF-state are different.
European Patent 0 053 854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215, Japanese Unexamined Laid Open Patent Application H09-266311 and Japanese Unexamined Laid Open Patent Application H10-223896 disclose semiconductor devices, which facilitate reducing the tradeoff relation between the on-resistance and the breakdown voltage. The drift layers of the disclosed semiconductor devices are formed of an alternating-conductivity-type drain drift layer including heavily doped n-type regions and heavily doped p-type regions arranged alternately. Hereinafter, the alternating-conductivity-type drain drift layer will be referred to sometimes as the xe2x80x9cfirst alternating conductivity type layerxe2x80x9d or simply as the xe2x80x9cdrain drift regionxe2x80x9d.
FIG. 10 is a cross sectional view of the vertical MOSFET disclosed in U.S. Pat. No. 5,216,275. Referring now to FIG. 10, the drift layer of the vertical MOSFET is not a uniform nxe2x88x92-type layer (impurity diffusion layer) but a drain drift region 22 formed of thin n-type drift current path regions 22a and thin p-type partition regions 22b laminated alternately. Hereinafter, the n-type drift current path regions will be referred to as the xe2x80x9cn-type drift regionsxe2x80x9d. The n-type drift regions 22a and p-type partition regions 22b are shaped with respective thin layers extending vertically. The bottom of p-type base region 13 is connected with p-type partition region 22b. The n-type drift region 22a is extended between adjacent p-type base regions 13 and 13. Although alternating conductivity type layer 22 is doped heavily, a high breakdown voltage is obtained, since alternating conductivity type layer 22 is depleted quickly by the depletion layers expanding laterally in the OFF-state of the MOSFET from the pn-junctions extending vertically across alternating conductivity type layer 22. Hereinafter, the semiconductor device which includes drain drift region 22 formed of an alternating conductivity type layer, which makes a current flow in the ON-state and is depleted in the OFF-state, will be referred to as the xe2x80x9csuper-junction semiconductor devicexe2x80x9d.
Although the channel stopper region in the breakdown withstanding region is of the same conductivity type with that of the drift layer usually, the channel stopper region is of the opposite conductivity type opposite to that of the drift layer sometimes depending on the manufacturing process. That is, the n-channel vertical MOSFET, the drift layer thereof is of n-type, includes a channel stopper region of p-type. In this case, the breakdown voltage of the MOSFET is stabilized by extending the channel stopper electrode connected to the outermost p-type region to the side of the active region so that the depletion layer in the peripheral portion of the MOSFET may not reach the outermost p-type region.
However, this structure causes a large leakage current in the n-channel super-junction MOSFET including an alternating conductivity type layer formed of p-type regions and n-type regions arranged alternately in the peripheral portion thereof, since the plural p-type regions of the alternating conductivity type layer connected to the plural p-type base regions in the active region is connected to one of the p-type regions in the channel stopper region. Increase of the leakage current causes not only increase of the losses in the OFF-state of the MOSFET but also breakdown of the MOSFET by thermal runaway.
In view of the foregoing, it would be desirable to provide a super-junction MOSFET reducing the tradeoff relation between the on-resistance and the breakdown voltage greatly and having a peripheral structure, which facilitates reducing the leakage current in the OFF-state thereof and stabilizing the breakdown voltage thereof.
According to a first embodiment of the invention, there is provided a semiconductor device including: a semiconductor chip having a first major surface and a second major surface facing opposite to the first major surface; an active region on the side of the first major surface; a layer with low electrical resistance of a first conductivity type on the side of the second major surface; a vertical drain drift region between the active region and the layer with low electrical resistance; the vertical drain drift region including a first alternating conductivity type layer formed of layer-shaped vertically-extending first regions of the first conductivity type and layer-shaped vertically-extending second regions of a second conductivity type, the first regions and the second regions being laminated alternately; a breakdown withstanding region between the first major surface and the layer with low electrical resistance, the breakdown withstanding region surrounding the vertical drain drift region, the breakdown withstanding region providing substantially no current path in the ON-state of the semiconductor device, the breakdown withstanding region being depleted in the OFF-state of the semiconductor device; the breakdown withstanding region including a second alternating conductivity type layer formed of layer-shaped vertically-extending third regions of the first conductivity type and layer-shaped vertically-extending fourth regions of the second conductivity type, the third regions and the fourth regions being laminated alternately; a fifth region of the first conductivity type around the second alternating conductivity type layer; and a sixth region of the second conductivity type around the fifth region. Advantageously, the fifth region separates the sixth region from the second alternating conductivity type layer. Advantageously, the fifth region is connected to the layer with low electrical resistance.
The semiconductor device according the invention facilitates reducing the leakage current, since the structure, which separates the sixth region from the second alternating conductivity type layer by the fifth region, interrupts the leakage current path. The structure, in which the fifth region is connected to the layer with low electrical resistance, facilitates stabilizing the breakdown voltage.
Advantageously, the semiconductor device according to the invention further includes an electrode connected electrically to the sixth region and arranged above the fifth region with an insulation film interposed between them. Advantageously, the electrode is extended at least over a part of the second alternating conductivity type layer with the insulation film interposed between them.
The electrode arranged as described above prevents depletion layers from reaching the sixth region of the second conductivity type under application of a reverse bias voltage and improves the reliability of the breakdown voltage.
Advantageously, a first pitch of repeating, at which a pair of the first region and the second region is repeated in the first alternating conductivity type layer, is wider than a second pitch of repeating, at which a pair of the third region and the fourth region is repeated in the second alternating conductivity type layer.
Advantageously, the first regions, the second regions, the third regions and the fourth regions are shaped with respective stripes in a horizontal plane. Advantageously, the boundaries between the first regions and the second regions in the first alternating conductivity type layer extend almost in parallel or almost in perpendicular to the boundaries between the third regions and the fourth regions in the second alternating conductivity type layer.
In other words, the planar stripes of the first regions and the second regions in the first alternating conductivity type layer are extended in parallel to or in perpendicular to the planar stripes of the third regions and the fourth regions in the second alternating conductivity type layer with no problem as far as the fifth region separates the sixth region from the second alternating conductivity type layer.
According to a second embodiment of the invention, there is provided a semiconductor device including: a semiconductor chip having a first major surface and a second major surface facing opposite to the first major surface; an active region on the side of the first major surface; a layer with low electrical resistance of a first conductivity type on the side of the second major surface; a vertical drain drift region between the active region and the layer with low electrical resistance; the vertical drain drift region including a first alternating conductivity type layer formed of layer-shaped vertically-extending first regions of the first conductivity type and layer-shaped vertically-extending second regions of a second conductivity type, the first regions and the second regions being laminated alternately; a very resistive region between the first major surface and the layer with low electrical resistance, the very resistive region surrounding the vertical drain drift region, the very resistive region being doped with an impurity of the first conductivity type and an impurity of the second conductivity type; a third region of the first conductivity type around the very resistive region; and a fourth region of the second conductivity type around the third region. Advantageously, the third region separates the fourth region from the very resistive region.
The semiconductor device according the second embodiment of the invention facilitates reducing the leakage current, since the structure, which separates the fourth region of the second conductivity type from the very resistive region by the third region of the first conductivity type, interrupts the leakage current path under application of a reverse bias voltage.